Send an exit code when the script terminates

This fixes the issue identified in #80 (comment) where even when the setting of `installed_paths` failed, the plugin would exit with exit code `0`.

Technical choices:

As PHPCS returns exit code `0`, even when the setting of the `installed_paths` failed, we need to verify that the paths we tried to set, have actually been set.

To that end, a new function `verifySaveSuccess()` has been added.
This function takes the paths which were to be saved and compares then to the paths which are actually set in PHPCS after the save to determine whether or not the plugin was successful.

A `0` exit code will be returned if successful, `1` if not.

src/Plugin.php

@@ -163,6 +163,7 @@ public function onDependenciesChangedEvent()
     {
         $io = $this->io;
         $isVerbose = $io->isVerbose();
+        $exitCode = 0;
 
         if ($isVerbose) {
             $io->write(sprintf('<info>%s</info>', self::MESSAGE_RUNNING_INSTALLER));
@@ -174,13 +175,16 @@ public function onDependenciesChangedEvent()
             $installPathUpdated = $this->updateInstalledPaths();
 
             if ($installPathCleaned === true || $installPathUpdated === true) {
-                $this->saveInstalledPaths();
+                $exitCode = $this->saveInstalledPaths();
             } elseif ($isVerbose) {
                 $io->write(sprintf('<info>%s</info>', self::MESSAGE_NOTHING_TO_INSTALL));
             }
         } elseif ($isVerbose) {
             $io->write(sprintf('<info>%s</info>', self::MESSAGE_NOT_INSTALLED));
+            $exitCode = 1;
         }
+
+        exit($exitCode);
     }
 
     /**
@@ -217,6 +221,8 @@ private function loadInstalledPaths()
      * @throws LogicException
      * @throws ProcessFailedException
      * @throws RuntimeException
+     *
+     * @return int Exit code of the PHPCS command.
      */
     private function saveInstalledPaths()
     {
@@ -249,9 +255,45 @@ private function saveInstalledPaths()
             $this->composer->getConfig()->get('bin-dir')
         );
 
-        if ($this->io->isVerbose() && !empty($configResult)) {
-            $this->io->write(sprintf('<info>%s</info>', $configResult));
+        $exitCode = $this->verifySaveSuccess();
+
+        if ($exitCode === 0) {
+            if ($this->io->isVerbose() && !empty($configResult)) {
+                $this->io->write(sprintf('<info>%s</info>', $configResult));
+            }
+        } else {
+            // Fail message.
+            $this->io->write('<info>FAILED</info>');
+        }
+
+        return $exitCode;
+    }
+
+    /**
+     * Verify that the paths which were expected to be saved, have been.
+     *
+     * @return int Exit code. 0 for success, 1 for failure.
+     */
+    private function verifySaveSuccess() {
+        $exitCode      = 1;
+        $expected      = $this->installedPaths;
+        $expectedCount = count($expected);
+
+        // Request the currently set installed paths after t.
+        $this->loadInstalledPaths();
+
+        if ($expectedCount === 0) {
+            if (count($this->installedPaths) === 0) {
+                $exitCode = 0;
+            }
+        } else {
+            $registered = array_intersect($this->installedPaths, $expected);
+            if ($expectedCount === count($registered)) {
+                $exitCode = 0;
+            }
         }
+
+        return $exitCode;
     }
 
     /**